The present invention relates to the fabrication of semiconductor integrated circuits (IC's). More particularly, the present invention relates to improved methods and apparatuses for igniting a plasma inside a plasma processing reactor.
During the manufacture of a semiconductor-based product, for example, a flat panel display or an integrated circuit, multiple deposition and/or etching steps may be employed. During the deposition step, materials are deposited onto a substrate surface (such as the surface of a glass panel or a wafer). Conversely, etching may be employed to selectively remove materials from predefined areas on the substrate surface.
There are many methods available to etch a substrate surface; one such method is plasma etching. In plasma etching, a plasma is formed from the ionization and dissociation of process gases. The positively charged ions are then accelerated towards the substrate where they drive the etching reactions.
One particular method of plasma etching uses an inductive source to generate the plasma. In order to create a plasma, a process gas is input into a plasma reactor chamber. Power is then supplied to an inductive coil using a RF power source, and a large electric field is produced inside the chamber. The electric field accelerates the small number of electrons present inside the chamber causing them to collide with the gas molecules. These collisions result in ionization and initiation of a discharge or plasma. The neutral gas molecules of the process gas when subjected to these strong electric fields lose electrons, leaving behind positively charged ions. These electrons are accelerated by the electric field produced by the induction coil and collide with other neutral molecules, causing more electrons to be ejected. This produces plasma breakdown and the plasma ignites.
To facilitate discussion, FIG. 1 a illustrates a prior art inductive plasma processing reactor 100. A typical inductive plasma processing reactor includes a chamber 102 with an antenna or inductive coil 110 disposed above a dielectric window 112. A substrate 114 is disposed above a chuck 116. The chuck 116 is disposed at the bottom of the chamber 102. When RF power is supplied to the inductive coil 110 an oscillating magnetic field 118 is created. This oscillating magnetic field 118 induces an electric current 120 with a plasma inside the chamber 102 and below the dielectric window 112. The electric current 120 flows in the opposite direction of the current in the inductive coil. The direction of the current is portrayed by the x and dot inside the inductive coil 110 and the electric current 120.
Typically, when RF power is supplied to the inductive coil 10 a voltage drop occurs across the dielectric window 112 and the vacuum chamber volume to electrically grounded surfaces. This voltage initiates the plasma breakdown. Generally, the free electrons are accelerated to a high energy by the circulating electric current 120. The electrons are accelerated in alternating directions (dependent on RF of the power source). The accelerated electrons collide with other neutral molecules creating more electrons and positively charged ions. As soon as the creation rate of free electrons exceeds their loss rate, the plasma ignites.
As can be appreciated by one skilled in the art, there are many different types of configurations for an inductive plasma processing reactor and conceptually they operate in a similar manner. By way of example, FIG. 1b is another illustration of an inductive plasma processing reactor wherein the coils are disposed around the chamber walls. An inductive plasma processing reactor 200 consists of a chamber 202 with an antenna or inductive coil 210 disposed around the chamber 202. A substrate 214 is disposed above a chuck 216. The chuck 216 is disposed above the bottom of the chamber 202. As discussed above in the description of FIG. 1a, when RF power is supplied to the inductive coil 210 an oscillating magnetic field 218 is created. This oscillating magnetic field 218 induces an electric current 220 within the plasma inside the chamber 202. The electric current 220 flows in the opposite direction of the current in the inductive coil. The direction of the current is portrayed by the x and dot inside the inductive coil 210 and the electric current 220.
Typically, the plasma is difficult to light, if the pressure is low inside the chamber. During this situation, the electrons inside the plasma have long mean free paths and may not collide with enough molecules before the electrons are lost to the chamber wall. This makes igniting the plasma very difficult. Shortening the mean free path of the electrons increases the number of collisions, which produces the chain reaction that sustains the plasma. The mean free path of the electrons can be shortened by increasing the pressure inside the chamber.
Changing the pressure has its drawbacks. By way of example, the optimal conditions for etching may be at low pressures. Therefore, if the pressure is increased to ignite the plasma, the best conditions for etching are not met. A two step process can be implemented to solve this problem by lowering the pressure after the plasma ignites under high pressure. In theory this would seem to work, except that it takes a finite amount of time to change from high to low pressure. During this time, the plasma environment changes while the substrate is present, which may produce undesirable and/or unpredictable etch results.
Also, the plasma is difficult to light if the voltage on the inductive coil is too low. In this situation, the electrons are not accelerated to a sufficiently high energy for ionization to occur. Increasing the voltage increases the energy of the electrons which results in more ionization inside the chamber.
Increasing the RF power to increase the voltages in the inductive coil has its limits. Increasing the RF power can damage or put undue wear on the electrical and mechanical components in the matching network, as well as placing additional electrical stress on other components of the RF power supply system. Also, at times it is not possible to get a high enough voltage to ignite the plasma, if the pressure is kept at the low pressure range that is called for by the etch process. Implementing a two-step process to use a high power level to light the plasma and decrease it to etch also has the aforementioned problem, i.e., the plasma environment is changed while the substrate is present. As mentioned, this change in condition may produce undesirable and/or unpredictable results.
In view of the foregoing, there are desired improved methods and apparatuses for igniting a plasma inside a plasma processing reactor without altering the preferred operating conditions, such as pressure and power.